Transmission system



g 1939- F. STRECKER ET AL 2,168,589

TRANSMISSION SYSTEM Filed July 14, 1936 5 51,45 coumoL ems CONTROL -U A N t a A Qu/c/r mass 62 rr e INVENTOR FELIX STRECKER BY KIMMEL ATTORNEY stopped, and this is attended with a disagreeable sence of voice or speech actions, at least one nings of signa1sor f even ,entire signals which,

will suppress the amplitude below a certain lim V adequately large value, the suppressor means will Patented Aug. s, 1939 M 2,168,589

UNITED {S ATES PATENT OFFICE TRANSMISSION SYSTEM Felix Strecker, Berlin- Siemensstadt, and Hermann Kimmel, Berlin-Charlottenburg, Ger- 'many, assignors to Siemens & Halske Aktiengesellschaft, Berlin, Germany, a corporation of Germany Application July 14, 1936, Serial No. 90,472 In Germany July 27, 1935 amplifiers may at the same time be used for voice amplification and not merely for blocking purposes, their use represents a progress of the art in contrast to the above-mentioned nonlinear resistances which introduce additional attenuation in the system.

This invention will best be understood by referring to the accompanyingdrawing, in which: Fig. 1 illustrates a curve of the tubes in a push-pull C-type amplifier;

Fig. 2 illustrates a curve of the tubes in a push-pull C-type amplifier having the working point shifted in a more negative direction than that shown in Fig. l;

Fig. 3 isa schematic diagram of the circuit of this invention.

The grid biasing voltage of C-type amplifiers, as is wellknown, is chosen in such a way that the working point lies outside the range in which plate current flows. Fig. 1, for instance, shows the characteristics a." and b of the tubes of a push-pull C-type amplifier. They illustrate the dependenceof the plate currentvia upon the grid potential eg. Theworking point is indicated by A. Now, an amplifier operated with such a biasing'potential possesses the property to complete- 13 suppress currents whose amplitudes do not go beyond. the value c. Larger amplitudes, say, amplitudes d which" essentially exceed the value c, are handledby the .C-type amplifier. If the amplifier "is' to be made impervious also for larger amplitudes with a view to insure echo killing, then'the working point for each tube must be shifted a corresponding distance in the negative sense as indicated in Fig. 2. It will be seen-thatalso amplitudes equal to 11 cause no changeinthe plate current. i

Itwillbe noted from an examination of Fig. 1, that when C-type amplifiers are used in the present connection, there happens a certain'nonlinear distortion; and the latter will be so much more marked, the more negative the biasing of the amplifiers,uand the smaller the transmitted amplitudes in comparison with the biasing potential. Hence, in choosing the biasing potential care must be taken so that the same will not be unduly or unnecessarily high. Inasmuch as the size of the biasing voltage, in downward direction, islimited by the noise level, the case may arise where the biasing voltage must be chosen higher than what would be permissible from the viewpoint of non-linear distortions. Now, in order that undistorted and faithful transmission may be secured also under these circumstances, this invention provides ways and means of a nature so-that, in the presence of 6 Claims.

This invention relates to an improved transmission system comprising amplifiers.

In transmission and repeating systems equipped with echo killers or suppressors, there exists the risk of a feedback being produced when the system is not acted upon by the voice. The said feedback may be caused by disturbing or stray currents which happen to penetrate the system. Inasmuch as these currents, as a general rule, are so feeble that the echosuppressor device will not be causedto operate instantaneously, a gradual building upby amplification of the stray'currents will be occasioned if the aggregate damping of the feedback path is negative. When the amplitude has reached an be caused to operate and to thereby interrupt the feedback path. The same process will be repeated again and again, that is to say, the echo suppressors are continually operated and background noise in the line. 7

Now, it'is known from the earlier art that. these'echosuppressors may-be combined with feedback stoppers in order that, also inthe absense of transmissison may be blocked. While it is true that in thismanner feedbacksare preveritable, there] results thisdrawbackthat, .because of the finite time of fresponse, the begin- 7 are notyet strong enough in orderto cause discontinuance of the blocking produced by the feedback block, .may be suppressed. Inasmuch as this action, under certain circumstances, may

liable to become extremely troublesome. It. has also been suggested in the prior artto insert in the feedback path resistances whose action is a function of the amplitude; and which iting value. If the said limitingvalue is chosen above the; disturbingor stray amplitudes. that may normally be expected,jthe risk of the arising of spurious feedbacks in the absence of voice actions is diminishable with means of the said sort.

Now, the present invention is concerned with a similar means adapted to secure the stability of operation when they system is not subject to voice actions, without, however, involving the drawback of requiring the insertion of distinct additional non-linear resistances. ,Thefdesired success, according tofthis" invention, isattained by the insertion andoonnection of C-type amplifiers in the 'feedback -path. Inasmuch as these voice transmission, the C-type amplifier in the transmitting sense by suitable circuit modifications and change of its grid potential, is altered so as to act as an A-type or a B-type amplifier. However, such a circuit change, if the stability of the feed-back circuit is not to be endangered, should be accomplished only when the opposite direction has already been rendered impervious by the operation of the echo suppressor device.

For the identical reasons it is recommendable that when the A or B-type amplifier after discontinuance of the voice transmission, is restored again to a C-type amplifier, this change should be accomplished with less retardation orlag than 5 the lag with which the blocking is rendered inoperative in the contrary direction. The change in the voltage is effected in both instances preferably by the aid of a mechanical relay or other known circuit means designed to impress upon and re- 20 movefrom the grid a definite and fixed amount of potential. 3

Fig. 3 shows an exemplified embodiment of the basic idea of the invention in schematic form. Between the two lines L1 and L2, by the aid of 25 balanced differential (or three-coil) transformers and the simulation or balancing means N1 and N2 is connected a two-wire intermediate telephone or through-line repeater. The latter comprises, in each sense or direction of transmission, ampli- 30 fiers V1 and V2 which are placed ahead of the push-pull C-type amplifiers C1 and C2. Above these latter are branched off amplifying rectifiers G1 and G2 whose direct current voltages serve for the purpose of shifting the grid potential of the 35 C-type amplifier of the opposite'direction of transmission. By the relays R1 and R2 which are also fed from the output current of the said rectifiers G1 and G2, the above-mentioned change of vthe C-type amplifiers into B or A-amplifiers is indicated. These relays which work with retarded response and rapid drop may be designed so that, for instance, part of thegrid biasing voltage impressed upon the C-type amplifiers is fishort-circuited for the purpose of eifecting their being changed, into A or B amplifiers. In this manner it will be an easy matter to shift the working point of the amplifiers a' certain,.pre-

arranged amount. It will be evident that, within thescope and spirit of thisinvention, it is possible also .to resortto other ways and-means adapted to insure a-prefixed and accurate shift of potential, say, circuit schemes discharge or glow-tubes. 5 The useof the idea underlying in connection with four-wire amplifiers is analogous and similar tothe use thereof with two-wire amplifiers as before. described. I

What is claimed is: 1. A transmission system for the suppression of echos comprising two single one way transmission paths for transmission of signals. in opposite comprising glowdirections connected in series and coupled be-' ::-,tween two separate incoming lines, a'sepaiate' 66 amplifier connected in series with'opposed outputs 'in each transmission path and ahead of' a push pull amplifier of the C-type whose grid bias voltage is' chosen in such a way that the working point lies outsidethe range in' which 70 the plate current flows, said push pull amplifier of the C-type connected intermediate said incoming lines to prevent feedback in theabsence of voice impulse action upon said system, a rectifier circuit whose DTC output voltage serves to shift, the grid potential gof said push pull amplifier this, invention branched from the output of said push pull amplifier with its output connected to the push-pull amplifier in the opposite path in such a manner as to render it inoperative as a C-type amplifier.

2. A transmission system for the suppression of echos comprising two single one way transmission paths for transmission of signals in opposite directions connected in series and coupled between two separate incoming lines, a separate amplifier connected in series with opposed outputs in each transmission path and ahead of a push-pull amplifier of the C-type whose grid bias voltage is chosen in such a way that the working point lies outside the range in which the plate current flows, said push pull amplifier of the C-type connected intermediate said incoming lines to prevent feedback in the absence of voice action upon said system, a rectifier circuit whose D-C output voltage serves to shift the grid potential of said push pull amplifier branched from the output of said push pull amplifier with its output connected to the push-pull amplifier in the opposite path in such a manner as to render it inoperative as a C-type amplifier, and a relay system connected to the output of said rectifier so that the echo suppression is effected by a shift of grid potential in said C-type push pull amplifiers. I

3. A transmission system for the suppression of echos comprising two single one way transmission paths for transmission of signals in opposite directions connected in series and coupled between two separate incoming lines, a separate amplifier connected in series with opposed outputs in each transmission path and ahead of a push pull amplifier of the C-type whose grid bias voltage is chosen in such a way that the working point lies outside the range in which the plate current flows, said push pull amplifier of the C-type connected intermediate said incoming lines to prevent feedback in the absence of voice action upon said system, a rectifier circuit whose D-C output voltage serves to shift the grid potential of said push pull amplifier branched from theoutput of said push pull amplifier with its output connected .to the push-pull amplifier in the opposite path in such a manner as to render.

it inoperative as a C-type amplifier and means tochange the working voltages of each of the pushpullC-type amplifiers, located in the direction acted'upon by the-voice impulses so as to substantially.decrease the.negative bias of said' push-pull amplifier.

4. A system according to claim 3, with the characteristic feature that the change of the negative bias on said push-pull amplifiers is produced with time-lag devices only when the opposite direction of transmission has been blocked by said rectifiers.

5. A system according to claim 3, with the characteristic feature that upondiscontinuance.

of the voice impulses the change from an amplie fier having a decreased negative bias into a C-type amplifier is effected more rapidly than the dis I FELIXSTRECKER. ,HERMANN KIMMELV 

